1. Field
This disclosure relates to the targeting of phosphatidylserine (PtdS) on cell membranes using Gla domain peptides and polypeptides. The use of these peptides and polypeptides as therapeutic agents is disclosed.
2. Related Art
Phosphatidlyserine (PtdS) is a negatively charged phospholipid component usually localized to the inner-leaflet (the cytoplasmic side) of the cell membrane. However, PtdS can be transported by scramblase (a member of the flippase family) from the inner-leaflet to the outer-leaflet and exposed on the cell surface. With very few exceptions, this active externalization of PtdS is a response to cellular damage (van den Eijnde et al., 2001; Erwig and Henson, 2008). For example, tissue injury signals platelets, leukocytes, and endothelial cells to rapidly and reversibly redistribute PtdS which leads to the promotion of coagulation and complement activation on cell surfaces. Similarly, apoptotic signals result in the externalization of PtdS however in a more gradual and sustained manner. This external PtdS provides a key recognition marker that enables macrophages to ingest dying cells from surrounding tissue while suppressing a full and detrimental immune response (Erwig and Henson, 2008). This removal process is essential for tissue homeostasis and in a “healthy” environment it is extremely efficient. In fact, despite the loss of >109 cells per day, the histological detection of apoptotic cells is a rare event in normal tissues (Elltiot and Ravichandran, 2010; Elltiot et al., 2009). However, there is evidence that in many pathological conditions the process of apoptotic cell removal is overwhelmed, delayed or absent (Elltiot and Ravichandran, 2010; Lahorte et al., 2004). For example several oncology studies suggest that a high apoptotic index is associated with higher grade tumors, increased rate of metastasis and a poor prognosis for the patient (Naresh et al., 2001; Loose et al., 2007; Kurihara et al., 2008; Kietselaer et al., 2002). These studies, and others like them, suggest that apoptosis and external PtdS expression can be a powerful marker of disease (Elltiot and Ravichandran, 2010).
There are several proteins with a high affinity for anionic phospholipid surfaces with Annexin-V being the most widely utilized as a PtdS targeting probe (Lahorte et al., 2004). With a high affinity for PtdS containing vesicles (Kd=0.5-7 nM) and a molecular weight (37 kDa) that falls below the threshold for kidney filtration (approx. 60 kDa) Annexin-V has shown promise in the clinic as an apoptosis-probe (Lin et al., 2010; Tait and Gibson, 1992). Moreover, it has been utilized for a wide range of indications including those in oncology, neurology and cardiology (Lahorte et al., 2004; Boersma et al., 2005; Blankenberg, 2009; Reutelingsperger et al., 2002). The use of biologic probes which target PtdS cell-surface expression has been shown both in vitro and in vivo. While their utility in the clinic is promising, they have, for the most part, not yet been exploited.